The cyclic AMP-responsive element binding protein (CREB) is documented to be overexpressed in leukemia, but the underlying mechanism remains unknown. Here, microRNAs (miRNA), which act as negative regulators of gene expression principally through translational repression, are investigated for the mediation of high CREB protein levels. A series of miRNAs that target CREB were identified. Real-time quantitative PCR revealed that miR-34b was expressed significantly less in myeloid cell lines, previously known for high CREB protein levels. Exogenous miR-34b expression was induced, and results revealed a direct interaction with the CREB 3 ¶-untranslated region, with the consequent reduction of the CREB protein levels in vitro. miR-34b restored expression caused cell cycle abnormalities, reduced anchorage-independent growth, and altered CREB target gene expression, suggesting its suppressor potential. Using reverse-phase protein array, CREB target proteins (BCL-2, cyclin A1, cyclin B1, cyclin D, nuclear factor-KB, Janus-activated kinase 1, and signal transducer and activator of transcription 3), as well as many downstream protein kinases and cell survival signaling pathways (AKT/mammalian target of rapamycin and extracellular signal-regulated kinase) usually elicited by CREB, were observed to have decreased. The miR-34b/miR-34c promoter was shown to be methylated in the leukemia cell lines used. This epigenetic regulation should control the observed miR34b expression levels to maintain the CREB protein overexpressed. In addition, the inverse correlation between miR-34b and CREB expression was found in a cohort of 78 pediatric patients at diagnosis of acute myeloid leukemia, supporting this relationship in vivo . Our results identify a direct miR-34b target gene, provide a possible mechanism for CREB overexpression, and provide new information about myeloid transformation and therapeutic strategies. [Cancer Res 2009;69(6):2471-8]

Key Points• MLL-AF6 leads to aberrant activation of RAS and its downstream targets.• RAS targeting is a novel potential therapeutic strategy in AML patients carrying t(6;11).A rare location, t(6;11)(q27;q23) (MLL-AF6), is associated with poor outcome in childhood acute myeloid leukemia (AML). The described mechanism by which MLL-AF6, through constitutive self-association and in cooperation with DOT-1L, activates aberrant gene expression does not explain the biological differences existing between t(6;11)-rearranged and other MLL-positive patients nor their different clinical outcome. Here, we show that AF6 is expressed in the cytoplasm of healthy bone marrow cells and controls rat sarcoma viral oncogene (RAS)-guanosine triphosphate (GTP) levels. By contrast, in MLL-AF6-rearranged cells, AF6 is found localized in the nucleus, leading to aberrant activation of RAS and of its downstream targets. Silencing MLL-AF6, we restored AF6 localization in the cytoplasm, thus mediating significant reduction of RAS-GTP levels and of cell clonogenic potential. The rescue of RAS-GTP levels after MLL-AF6 and AF6 co-silencing confirmed that MLL-AF6 oncoprotein potentiates the activity of the RAS pathway through retention of AF6 within the nucleus. Exposure of MLL-AF6-rearranged AML blasts to tipifarnib, a RAS inhibitor, leads to cell autophagy and apoptosis, thus supporting RAS targeting as a novel potential therapeutic strategy in patients carrying t(6;11). Altogether, these data point to a novel role of the MLL-AF6 chimera and show that its gene partner, AF6, is crucial in AML development. (Blood. 2014;124(2):263-272) IntroductionThe mixed lineage leukemia (MLL) protein is a histone H3 lysine 4-specific methyltransferase, commonly associated with transcriptional activation.1 MLL is essential for both embryonic development and normal hematopoiesis, mainly through transcriptional regulation of the homeobox (HOX) gene.2 Chromosome translocations involving MLL locus are 1 of the major genetic lesions leading to acute leukemia. MLL translocations are detected in up to 80% of infant acute leukemia and in approximately 10% to 15% of childhood acute myeloid leukemia (AML).3,4 Aberrant proteins resulting from translocations, duplications, or amplifications of the MLL gene cause alteration of the differentiation program with severe effects on leukemogenesis. 5,6 To date, more than 60 fusion partners of MLL have been described, which result in AML, acute lymphoid, and biphenotypic or chemotherapy-related leukemias. 7,8 The underlying mechanisms for MLL-mediated leukemogenesis have been extensively studied; however, they still remain elusive for many of the described translocations. MLL-rearranged AML is, in fact, a heterogeneous disease, which depends on the MLL partner gene for its biological and clinical features, such as gene expression and genomic imbalances. 4 Among diverse fusion genes, the 1 that has been consistently associated with the worst outcome both in adult and pediatric AML is MLL-AF6. 9The t(6;11)(q27;q23) translocatio...

Moreover, for eight patients with mutations in IKZF1 (n ¼ 3), RUNX1 (n ¼ 3), ASXL1 (n ¼ 1), WT1 (n ¼ 2) and IDH1 (n ¼ 2), matched DNA samples from initial diagnosis at chronic phase were available. In none of the chronic phase CML samples were the respective IKZF1 deletions or RUNX1 and ASXL1 mutations detectable, indicating that mutations in IKZF1 and RUNX1 were acquired at the time of transformation to BC-CML, and thus act as driver mutations in these cases. In contrast, WT1 and IDH1 mutations were detected at diagnosis in chronic phase in one case each.With respect to clinical data, associations with survival for RUNX1, ASXL1, IKZF1 and WT1 alterations were investigated. No molecular parameter was significantly associated with outcome, which may be due to the short median survival in BC-CML (n ¼ 34 patients with survival data available; median overall survival: 9.3 months).In conclusion, the aberrant BCR-ABL kinase causes genomic instability of the CML clone by inefficient DNA repair, resulting in chromosomal alterations and molecular aberrations of transcription factors. This study on 12 genes demonstrated for the first time that in 76.9% of the BC-CML patients, molecular mutations are detectable. The high mutation rate of RUNX1 (33.3%), ASXL1 (20.5%) and IKZF1 (17.9%) represented important molecular abnormalities in the progression of CML. In particular, IKZF1 and RUNX1 alterations, both involved in cell differentiation, were identified as important markers of disease progression from chronic phase to BC. Although this is a comprehensive study, further investigations are required to identify additional pathogenetic alterations, as in four cases (10.2%) of our cohort no chromosomal or molecular genetic alterations were observed in addition to t(9;22)(q34;q11). Conflict of interestCH, S Schnittger, WK, and TH have equity ownership of MLL Munich Leukemia Laboratory. VG, AK, MZ, CE, S Schindela, and SW are employed by MLL Munich Leukemia Laboratory. MCM and AH declare no conflict of interest.

ARTICLEShaematologica | 2013; 98(4) Molecular Aspects of LeukemiaMicroRNA-34b down-regulation in acute myeloid leukemia was previously shown to induce CREB overexpression, thereby causing leukemia proliferation in vitro and in vivo. The role of microRNA-34b and CREB in patients with myeloid malignancies has never been evaluated. We examined microRNA-34b expression and the methylation status of its promoter in cells from patients diagnosed with myeloid malignancies. We used gene expression profiling to identify signatures of myeloid transformation. We established that microRNA-34b has suppressor ability and that CREB has oncogenic potential in primary bone marrow cell cultures and in vivo. MicroRNA-34b was found to be up-regulated in pediatric patients with juvenile myelomonocytic leukemia (n=17) and myelodysplastic syndromes (n=28), but was down-regulated in acute myeloid leukemia patients at diagnosis (n=112). Our results showed that hypermethylation of the microRNA-34b promoter occurred in 66% of cases of acute myeloid leukemia explaining the low microRNA-34b levels and CREB overexpression, whereas preleukemic myelodysplastic syndromes and juvenile myelomonocytic leukemia were not associated with hypermethylation or CREB overexpression. In paired samples taken from the same patients when they had myelodysplastic syndrome and again during the subsequent acute myeloid leukemia, we confirmed microRNA-34b promoter hypermethylation at leukemia onset, with 103 CREB target genes differentially expressed between the two disease stages. This subset of CREB targets was confirmed to associate with high-risk myelodysplastic syndromes in a separate cohort of patients (n=20). Seventy-eight of these 103 CREB targets were also differentially expressed between healthy samples (n=11) and de novo acute myeloid leukemia (n=72). Further, low microRNA-34b and high CREB expression levels induced aberrant myelopoiesis through CREB-dependent pathways in vitro and in vivo. In conclusion, we suggest that microRNA-34b controls CREB expression and contributes to myeloid transformation from both healthy bone marrow and myelodysplastic syndromes. We identified a subset of CREB target genes that represents a novel transcriptional network that may control myeloid transformation.

The inducible cyclic AMP (cAMP) early repressor (ICER) and cAMP response element-binding protein (CREB) are transcriptional regulators of the cAMP-mediated signaling pathway. CREB has been demonstrated to be upregulated in the majority of childhood leukemias contributing to disease progression, whereas ICER, its endogenous repressor, was found to be downregulated. Our research focus has been the function of restored ICER expression. ICER exogenously expressed in cell lines decreases CREB protein level and induces a lowered clonogenic potential in vitro. It decreases the ability of HL60 to invade the extramedullary sites and to promote bone marrow angiogenesis in nonobese diabetic-severe combined immunodeficient mice, demonstrating its potential effects on tumor progression. ICER represses the majority of 96 target genes upregulated by CREB. It binds CRE promoters and controls gene expression restoring the normal regulation of major cellular pathways. ICER is subjected to degradation through a constitutively active form of the extracellular signal-regulated protein kinase, which drives it to the proteasome. We propose that ICER is downregulated in HL60 to preserve CREB overexpression, which disrupts normal myelopoiesis and promotes blast proliferation. These findings define the function of ICER as a tumor suppressor in leukemia. Unbalanced CREB/ICER expression needs to be considered a pathogenetic feature in leukemogenesis. The molecular characterization of this pathway could be useful for novel therapeutic strategies.